1. Field of the Invention
The present invention relates to a hydraulic system for extending and retracting a telescopic boom, such as provided in a crane, and, more particularly, to such a system adapted to retract the boom quickly.
2. Description of the Prior Art
In a typical hydraulic system to extend and retract a telescopic boom, two hydraulic motors are provided: a first motor connected between the base boom section and the mid boom section (which motor may be identified as the mid boom telescoping motor) and a second motor connected between the mid boom section and the tip boom section (which motor may be identified as the tip boom motor). Each motor usually consists of a cylinder with a piston slidably received therein to define two chambers in the cylinder: an extend chamber in which fluid under pressure is received to extend one boom section to which the motor is connected with respect to the other connected boom section, and a retract chamber in which fluid under pressure is received to retract said one boom section with respect to said other boom section. Each piston has a piston rod connected thereto which extends from the cylinder. It has been known heretofore to provide two passages through each piston rod, one to the extend chamber of the cylinder in which the piston is contained and the other to the retract chamber of that cylinder. A control valve is generally provided which has a first valve portion to control the first hydraulic motor and a second valve portion to control the second hydraulic motor.
In the usual installation, the piston rod of the first, or mid boom, motor is secured to the base boom section (which pivots but does not move axially). Consequently, the piston rod of the first motor does not move axially, and the two passages extending therethrough can be conveniently connected to the first valve portion to control the flow of pressure fluid to, and the flow of return fluid from, the extend and retract chambers of the first motor. The cylinder end of the first hydraulic motor is connected to the mid boom section for extension thereof when the first motor is extended.
The rod end of the second, or tip boom, motor is generally connected to the mid boom section (for movement therewith when the mid boom section is extended by extension of the first motor), and the cylinder end of the second motor is connected to the tip boom section for extension thereof relative to the mid boom section when the second hydraulic motor is extended.
Since the rod end of the second motor moves axially (although not to the extent of the cylinder end), the passages through the piston rod must be connected to the second valve portion (for the supply of fluid to and the return of fluid from the extend and retract chambers of the motor) by means of flexible hoses. It is also customary to provide a reel mounted between the second motor and the second valve portion to store a portion of the flexible hose when the mid boom section is retracted, and to pay out said portion of flexible hose when the mid boom section is extended.
A significant problem encountered with the prior art hydraulic telescoping system for a boom lies in the excessive time it takes to retract the boom, and particularly the tip boom section thereof. It is the solution of this problem to which the present invention is directed.